Devices known as pigs are commonly passed through conduit, such as pipelines, in order to service or inspect the pipeline. A pig may contain the necessary servicing or inspection equipment or may draw other devices behind it. In pipelines for conveying fluids, the pig is commonly driven along the pipeline by the flow of the pipeline fluid. To achieve this, the pig is usually fitted with two spaced apart flexible or expandable cups that are attached to a rigid body of the pig and extend outwardly to the interior of the conduit wall. The cups are adapted to provide a sufficient seal in the pipeline permitting the pig to travel at the speed of the pipeline fluid.
In many instances, the pig comprises more than one section with a towing section having an elongated rigid body supporting the expandable cups. The towing section or device pulls other inspection devices through the pipeline. These other inspection devices may also clean the pipeline.
In some instances, pigs may become stuck in pipelines due to bends in the pipeline that have a radius equal to the diameter of the pipeline. In these instances the pig is too long and straight to negotiate the bend in the pipeline. In other instances the pig may become stuck because it loses its seal with the conduit due to openings in the pipeline wall that are larger than the length or distance between cups of the pig. These openings occur where the conduit has intersections with other pipes. These intersections are referred to as full bore T's and Y-connections. In particular the full bore T intersection occurs when a conduit intersects the pipeline at 90 degrees. The pig must be designed to negotiate past both bends and intersections in the pipeline which two factors have opposite requirements for the pig. Clearly a longer pig works better to maintain a seal with the pipeline as it negotiates past open T intersections. On the otherhand, a shorter pig is able to negotiate past bends in the pipeline. Consequently, pipelines having both the bends and intersections can increase the risk of a pig becoming stuck in the pipeline making the pipeline “unpiggable”.
At the present time pig towing devices have collapsible multi diameter flexible cups which allow for movement of the cups relative to each other and the axis of the rigid body of the pig towing device to permit the pig towing device to negotiate slight bends in the pipeline. One such pig having a multi diameter flexible cup is disclosed in more detail in my U.S. Pat. No. 6,381,797 issued May 7, 2002. Even so, the multi diameter cups of the pig towing devices must still be spaced sufficiently far enough apart so as to allow each one of the cups to maintain a seal with the conduit as the pig negotiates past intersection openings in the pipeline wall. That is, the cups must be of spaced apart a sufficient distance to permit the trailing cup to push the leading cup via the rigid interconnecting body past the intersection and then permit the leading cup to drag the trailing cup via the rigid interconnecting body past the intersection so that the pig towing device does not become stuck in the pipeline. However, the length of the rigid elongated body interconnecting the two cups still restricts negotiation of the pig towing device around tight bends or curves within the pipeline.
Accordingly, there is a need for the development of a pig towing device which is able to negotiate past intersections within the pipeline as well as being able to negotiate through curves or bends in the pipeline.